Circulating fluidized bed combustion (FBC) reactors are being utilized in an increasing number of applications including the generation of steam wherein fossil fuels such as coal are used as a fuel source. FBC reactors typically utilize fuels having a lower energy content than are used in standard boiler furnaces. FBC reactors are favored over conventional boiler systems because the combustion flue gas can be desulfurized economically before being emitted into the atmosphere.
In a typical process for producing and recovering heat energy which employs a circulating FBC reactor, a solid fuel and an oxygen-containing gas are introduced into a reactor or furnace and combusted to form hot exhaust gases containing ash particles. The ash-containing exhaust gas is discharged from the furnace and cooled by indirect heat exchange against the water-containing tubes surrounding the reactor to form a cooled exhaust flue gas thereby providing energy for generating steam or heating water. Heat exchange is typically effected by lining the walls of the reactor with tubes or lines carrying water. Such tubes or lines are capable of capturing a significant portion of the heat generated during combustion by such indirect heat exchange. A portion of the solid particles entrained in the exhaust gas is separated and collected for disposal while an additional portion is recycled into the combustion unit.
Heat transfer between the hot flue gas containing ash particles and the cooling tubes occurs by a combination of radiant and convective cooling. Radiation is an especially important form of heat transfer in high temperature systems. Normally, in combustion processes, the gaseous products radiate heat to the heat transfer surfaces. Radiant heat transfer becomes less efficient as the combustion gases cool and convection becomes the dominant heat transfer mechanism.
In the case of FBC reactors, radiant heat transfer from solid particles to the heat transfer surfaces is the predominant mechanism although convection plays an important role. Radiant heat transfer from the gaseous combustion products plays only a minor role in the process. Proper heat transfer is dependent upon maintaining an adequate concentration of ash particles within the reactor which promotes mixing and heat transfer. If the average ash particle size is too small, ash tends to fly out of the reactor causing a rise in temperature which negatively impacts on the efficiency of pollutant removal.
The combustion of certain coals produces ash having very fine particles known as fly ash which tend to be blown out of the reactor bed and the accompanying cyclones. The loss of such fly ash causes the particle density in the reactor to drift below the operating range which is most effective for promoting heat transfer rendering a reactor temperature which is too high to conduct efficient removal of pollutants. Consequently, the SO.sub.x and NO.sub.x content of the flue gas may exceed permissible levels established by federal and state governments which could result in assessment of penalties and/or imposition of an injunction against the continued operation of the plant.
Several approaches have been proposed for enhancing heat transfer between the heated flue gas containing ash particles and the water-containing tubes surrounding the reactor. For example, bottom ash classification and recycling are practiced to maintain the particle density in the combustion zone within the desired size range. Excess limestone beyond the amount necessary to effect desulfurization may be added into the combustion reactor to enhance heat transfer. However, this solution is not entirely satisfactory because calcined limestone catalyzes NO.sub.x formation from nitrogen containing fuel sources thereby creating an additional pollutant.
Sand addition has been used as an inert bed in the startup of FBC reactors and can be used to maintain combustion bed density. While sand is adequately inert and does not suffer unacceptable attrition during the combustion process, sand is extremely erosive and can cause unacceptable wear of the pipes, heat transfer surfaces and refractory surfaces within the reactor. Consequently, the increased down time of plants required for repair of eroded surfaces has made the use of sand addition very undesirable.
U.S. Pat. No. 3,645,237 discloses a FBC system for use in commercial boiler systems. An order-of-magnitude increase in heat transfer rate is achieved by utilizing a bed of inert material (such as sand) at velocities which cause the bed to become fluidized. Suitable materials are granular particles ranging in size from 0.001 in. (25 .mu.m) to about 0.10 in. (2500 .mu.m) in diameter. Particles of roughly 46 mesh (0.0013 in., 32 .mu.m)) are generally preferred. The particles are refractory materials which withstand very high temperatures without fracturing or crumbling during the process. Suitable particles include various metal alloys, ceramic material and other inert materials such as alumina, zirconia or mullite.
U.S. Pat. No. 4,157,245 discloses a process for gasifying a solid carbonaceous material in a gasification zone wherein such zone includes means for substantially impeding vertical back mixing of vertically moving solids which comprises introducing a heat transfer material into an upper portion of the gasification zone. A preferred heat transfer material is sand.
U.S. Pat. No. 4,704,084 discloses a method of lowering nitrogen oxides to a desired level and minimizing sulfur dioxide in the reaction gases formed during combustion of fuel in a FBC reactor. The FBC reactor has a lower dense fluidized bed of relatively large particles, an upper dispersed entrained bed of relatively fine particles recirculating through the dense fluidized bed and an entrained sulfur sorbent material therein. The method comprises the steps of:
(a) operating a lower region of the multisolid fluidized bed under substoichiometric conditions such that NO.sub.x is reduced to the desired level;
(b) operating an upper region of the multisolid fluidized bed above the substoichiometric lower region under oxidizing conditions to complete the combustion of the fuel;
(c) maintaining a size difference between the relatively large particles and the relatively small particles such that substantially all of the large particles are at least four times the size of substantially all of the small particles; and
(d) recycling a portion of the relatively fine particles through the lower region of the dense bed operation under substoichiometric conditions and a portion of the relatively fine particles from the entrained bed through substantially only the upper region which is operating under oxidizing conditions thereby depressing the temperature of such oxidizing region to a level conducive to sulfur capture by the sulfur sorbent material and operating the lower region at as low a temperature as will support combustion.
U.S. Pat. No. 4,771,712 discloses a method of burning solid fuel containing low melting point alkaline compositions such as alkali metal salts. A fuel such as lignite or salty brown coal is introduced into the reaction chamber of a circulating fluidized bed reactor and is mixed prior to introduction to the reaction chamber with a reactant material capable of reacting with the low melting point alkaline compositions of the fuel to produce a high melting point alkali metal compound during combustion. The temperature in the reaction chamber is kept below the melting point of the formed alkali metal compounds. The reactant material comprises silicon dioxide, an oxide, metal oxide or hydroxide of aluminum, calcium, magnesium, iron, titanium or a mixture of two or more thereof. Kaolin is particularly effective when the molar ratio of Al/Na and K is at least 1.0.